Strengthened arc carbon electrode joints



I Dec. 22, 1970 ROCHE, JR 3,549,739

STRENGTHENED ARC CARBON ELECTRODE JOINTS Original Filed May 12; 1966INVENTOR MARTIN A. ROCHE,JR.

ATTORNEY United States Patent 3,549,739 STRENGTHENED ARC CARBONELECTRODE JOINTS Martin A. Roche, Jr., Brook Park, Ohio, assignor toUnion Carbide Corporation, a corporation of New York Continuation ofapplication Ser. No. 549,670, May 12, 1966. This application Oct. 1,1968, SH. N0. 764,048 Int. Cl. B28b 11/08 US. Cl. 264-162 2 ClaimsABSTRACT OF THE DISCLOSURE A method of effecting an improved bondbetween a core and a shell in an arc carbon electrode. The shell isprovided with a plurality of ribs along its inside surface and the coreis pressed within the shell and between the ribs. A cement is thenforced between the shell and the core under pressure.

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 stat.435; 42 U.S.C. 2457).

This application is a continuation of application Ser. No. 549,670 filedMay 12, 1966.

This invention relates to arc carbon electrodes and more particularly toare carbon electrode joints which have been strengthened to withstandhigh torsional forces.

Arc carbon electrodes are employed in devices wherein the are which isstruck between a positive carbon and a negative consumable ornonconsumable cathode is employed as a source of illumination. Thesedevices generally require that the arc function continuously for anindefinite duration of time as, for example, in a solar simulationdevice, or that it function continuously for a definite period of timesuch as in certain types of searchlight apparatus. Providing sufficientlengths of carbon elec trodes to sustain an are for the desired durationis ac complished by joining electrodes together. This method, in effect,produces endless carbons and may be accomplished in practice withoutinterrupting the are by threading a new carbon into the nonburning endof another carbon before the latter is entirely consumed by the arc.

A variety of carbon electrode compositions and constructions arecombined in an attempt to improve the desired characteristics of aparticular are. A commonly known and often used electrode, for example,comprises a carbonaceous shell and a core each of which may be composedof a number of suitable materials. The thickness of the shell is animportant consideration in this type of electrode. Normally, thediameter of the core is approximately /3 the outside diameter of theelectrode. The shell thickness therefore is usually about /6 thediameter of the electrode. Electrodes which are constructed entirely ofcore material are not used since the core material includes metal saltswhich consume rapidly and develop a gas envelope (plasma) which must becontained by the shell in order to sustain arc operation.

Another important consideration in carbon electrodes which are composedof a core and a shell and which are to be joined together end to end ashereinbefore described is the strength of the bond between the core andshell. A weak union of shell and core often results in an easy breakageof the joint which is formed by connecting two electrodes end to end. Inan effort to increase the strength of the connection between the shelland core, a wide variety of cements have been used. These cements areusually applied to the outer-surface of the core or on the insidesurface of the shell prior to inserting the core into the shell.

3,549,739 Patented Dec. 22, 1970 It is the primary object of thisinvention to provide a method whereby the strength of the core-to-shellbond in an arc carbon electrode is increased.

It is another object of this invention to provide a strengthened jointwhen are carbon electrodes are connected end to end.

Other objects and advantages of the invention will become apparent fromthe following description, taken in conjunction with the followingdrawing wherein:

FIG. 1 is a partial cross-sectional view of arc carbon electrodes whichhave been joined end to end; and

FIG. 2 is a section taken along lines 22 of FIG. 1.

Broadly, the objects of this invention are accomplished by forcing acement under pressure into the void between core and shell of an arccarbon electrode. The cement employed may be any of the many suitablecements presently used in the art for core to shell bonding. Most of thecements which are used for this purpose have a low viscosity and maytherefore be readily forced between core and shell at a pressure lessthan that at which the shell would rupture. These cements, however, losetheir bonding characteristics when they are pyrolyzed to hightemperatures. Yet, if the cements are not pyrolyzed to thesetemperatures, condensible volatiles will form during the period when theelectrode is being consumed in the arc and will attack the electricalcontacts in the illumination device. Therefore, it is preferable toemploy cement systems which retain adequate bonding strength after beingpyrolyzed at high temperatures. One such cement is disclosed in US.application, Ser. No. entitled, Strengthened Arc Carbon Electrode Joint,filed concurrently herewith. Such a cement contains relatively largeamounts of hardeners and inert fillers which increase the viscosity ofthe cement system to the level where it becomes impractical to force thecement between a core and a shell since the shell will rupture. Inaccordance with this invention, therefore, best results are achievedwhen core and shell are substantially equally spaced prior to forcingthe cement therebetween. The preferred method of the invention includesthe placing of a plurality of ribs on the inside surface of the shell ofan arc carbon electrode. The core when placed inside a ribbed shell is,if the ribs are properly positioned, centered with respect to the insidediameter of the shell and thus a cement may be forced uniformly betweenthe shell and core. In this manner, cement is symmetrically interposedaround approximately 360 of the core surface and a rigid bond isinsured.

Referring now to FIG. 1, a carbonaceous electrode 10 comprising an outershell 12 and an inner core 14 is in direct communication with a similarcarbonaceous electrode 16. The electrode 16 has a threaded protrusion 18which fits into a threaded recess 20 in the other electrode 10. On theoppoite ends of these electrodes 10 and 16 are a threaded protrusion 22and a threaded recess 24 respectively thereby providing each electrodewith a protrusion at one end and a recess in the other end. A cement 26is interposed between the outer shell 12 and the inner core 14.

In FIG. 2, the shell 12 is illustrated as having a plurality of ribs 28which extend along its inner surface 30. The ribs 28 are in contact withthe periphery of the core 14 at various points 32. The cement 26 isforced into place in the annular space 34 between the core 14 and theinner surface 30 of the shell 12. The ribs properly align the peripheryof the core 14 with respect to the inner surface 30 of the shell 12. Asa result, the cement 26 is symmetrically distributed about the entireperiphery of the core. Thus, a strong bond is efiected between the core14 and the shell 12.

The ribs are preferably about .006" deep and .030" wide at their baseand preferably extend along the entire length of the shell. Ideally, astraight core and shell with true roundness require only three ribsdisplaced approximately 120 to center the core. Because of the nominaltolerances of the dimensions of the shell and core, additional ribs aregenerally required. It has been found that six ribs spaced approximately60 apart on the inside surface of the shell are sufficient. It will beappreciated that variations in the number of ribs may be employedwithout adversely affecting the strength of the bond achieved. For bestresults, the ribs are substantially equally spaced about the insidesurface of the shell.

During assembly, the carbonaceous shell is clamped while a core isplaced therein. A cement is then forced into the annular space betweenthe core and shell under a pressure sufficient to cause the cement toflow into the space, but less than the rupture strength of the shell.The pressure to be employed will depend on a number of factors such asthe viscosity of the cement, the size of the annular space into whichthe cement must flow, the desired depth of penetration of the cementwithin the electrode, the rupture strength of the shell, etc. It will beappreciated, however, that it is well within the skill of an artisan todetermine a suitable pressure to employ. In addition, any suitabledevice may be used to apply the required pressure such as a standardgrease gun or a positive displacement pump. After the cement isinterposed between core and shell, it is cured and then pyrolyzed insitu.

In order to test the effectiveness of the invention, several electrodeseach having a non-ribbed shell which was bonded to a core by a cementforced therebetween at a pressure of 1,000 p.s.i. by means of a greasegun were joined end to end in the manner illustrated in had beenpyrolyzed at 10001400 C.; the core was composed of natural graphite andmetal salts bonded together with a coal tar pitch which was pyrolyzed at750-1000" C. The resinous cement which was employed in each of theelectrodes was composed of percent diglycidyl diether of bisphenol F, 25percent BRP-5012 (a powdered phenol resin manufactured by Union CarbideCorporation, New York, N.Y.) Bakelite 1 phenolic resin, 12.5 percentfurfuryl alcohol and 12.5 percent furfuryl aldehyde. The cement in theassembled electrodes was cured in air for eight hours at 110 C.

After the cement had been cured, the carbon electrode sections weremachined to provide protrusions and recesses in such a manner thatjoints could be formed by the threading of a protrusion into a recess.The protrusions and recesses were fitted with 18 NS truncated formsattainable with standard taps and dies. The strength of the core toshell bond was determined by measuring the torque which was required tobreak the joints. It has been found that in joints formed by y ;l8 NStruncated thread electrodes having an adequate bond between the shelland core, the protrusion exceeds the strength of the recess to which itis joined. Therefore, when the torque which is required to break suchjoints is generated, the point of failure should be in the shell wall ofthe recess. The location of the break is then an indication of thestrength of the bond between the core and shell. If the recess cracks,the strength of the core to shell bond is adequate. If, on the otherhand, the shell wall on the protrusion breaks away from the core, thestrength of the bond is inadequate. The following table lists the torquewhich was required to break the joints on each group of the samplestested.

TABLE I.TORQUE REQUIRED TO BREAK JOINTS Cement forced Cement intoannular painted on space between Test Number cores, torque core andshell,

in.oz. Type of breakage torque in.-oz. Type of breakage 255 Recess broke290 Recess alone broke. 235 Protrusion broke 265 Do. 245 Recess broke,protrusion cracked 275 Do. 230 Recess broke 205 Do. 290 o 320 Do. 270Recess broke, protru n acke 275 Do. 270 Recess broke 280 Do. 250 Recessbroke, protrusion crackcd 305 Do. 250 Recess broke 380 Do. 250 do 275Do. 11 265 D0.

FIG. 1 and tested for torsional breaking strength. The cement penetrateda depth of approximately 3 inches along each end of the electrodes.Several other electrodes also having nonribbed shells, but having thesame cement interposed between the core and shell by painting thesurface of the core with the cement prior to insertion into the shell,were similarly tested.

The test equipment consisted of a turret lathe with the turret replacedby torque transmitting and sensing devices which were mounted on thelathe ways. The torque transmitting members included a collet chuck anda connecting shaft cradled by two bearings which were coaxially mountedwith the lathe spindle. The connecting shaft was joined directly to theshaft of a bracket-mounted torque sensing device. A strain gaugerecorder amplified the signal from the electrical resistance straingauge of the torque sensor and recorded the results on a chart. Theconnecting shaft was designed to allow the collet chuck to travelaxially during the assembling and testing of a joint.

The electrode sections which were tested were 5" long and includedshells with a wall thickness of approximately .090" and a diameter ofapproximately .630". The shells and cores were composed of the followingmaterials: the shell was composed primarily of artificial graphitebonded with a coal tar pitch which As indicated in the table, the jointswhich were formed with the carbon electrodes having the cementinterposed between shell and core by pressure injection were muchstronger than those which were formed by electrodes having cementinterposed by the conventional method of painting the core. The averagetorque required to break the joints on electrodes formed by theinjection method is higher than on those formed on electrodes preparedby the painting method. Most importantly, there were no failures due tothe protrusion cracking or breaking when the injection method was used.Thus, it is readily apparent that electrodes which are assembled inaccordance with the principles of this invention are clearly superior tothose which are constructed in a conventional manner.

What is claimed is:

1. A method of fabricating a joinable arc carbon electrode having a coreand shell comprising:

(a) providing said shell with a plurality of inwardly protruding ribsextending along the inside surface of said shell;

(b) placing said core within said shell and centering said core thereinby having the outer surface of the core contact substantially all theprotruding ribs thereby providing longitudinal cavities each of whichRegistered trademark of Union Carbide Corporation, New York, N.Y., usedfor a group of resinous and plastic materials.

is defined by a pair of adjacent ribs, the outer surfaces of the coreand the inner surface of the shell;

(0) forcing a cement into said cavities under pressure sufficient tocause said cement to flow therein but less than the rupture strength ofthe shell;

(d) curing said cement to effect a bond between the shell and core;

(e) machining one end of the electrode to provide an internally threadedrecess; and

(f) machining the other end of the electrode to provide an externallythreaded protrusion.

2. The method of claim 1 wherein the steps of placing said ribssubstantially equally spaced about the inside surface of the shell andextending said ribs along substantially the entire length of the shellare added.

References Cited UNITED STATES PATENTS 2,726,357 12/1955 Sachs28720.92EX

3,048,433 8/1962 Doetsch 287127E 5 3,313,476 4/1967 Lauzau et a1287l27EX FOREIGN PATENTS 773,579 5/1957 Great Britain 287l27E 10 DAVIDJ. WILLIAMOWSKY, Primary Examiner W. L. SHEDD, Assistant Examiner US.Cl. X.R.

